Alloy windows are normally either fastened in with self tapping screws or interscrews ( a screw on the outside that goes into a matching socket held in a backing ring on the inside.
Depending on what type of sealant has been used they can be pigs to get out. You will need something like a wide old chisel or stiff stripping knife ground sharp to break the seal then put wedges in and work round the window adding more wedges as you go. Hope they are not set in sikaflex or they will likely rip the gelcoat off & come out beyond repair.
Possibly the window unit is fitted from outside with a trim around them on the inside all through bolted with interscrews. If so remove the interscrews and the trim on the inside then remove the window unit from outside.
The Hadlow info sheets look spot-on. That is almost exactly the procedure I have used for Sadler windows. Slight difference in the method recommended for re-applying the sealant from the one I use, but good advice for the OP.